The invention pertains to a device for heating preforms consisting of thermoplastic material.
Devices of the class in question are usually provided in stretch blow-molding machines. They serve to heat the plastic preforms consisting of, for example, polyethylene terephthalate (PET), from which, after the heating step, the desired plastic containers are produced by blow-molding.
As a rule, devices of the class in question comprise an endless conveying device, to which the preforms are supplied by a star wheel feeder. So that the preforms can be thermally conditioned, the conveying device then conveys them through a heating channel extending over a defined section of the transport route. The preforms or the parts of the preforms which extend into the heating channel are heated to temperatures above the glass transition temperature of the material to be processed.
Other types of devices are also known, of course, such as those in which the preforms are stationary while being heated.
In devices of the class in question, the heating channel can be formed by, for example, at least one heating module; or, when the preforms are heated while in motion, it is usually formed by several heating modules connected to each other in the transport direction, each of which forms one section of the heating channel. Heating modules of this type are described in, for example, DE 10 2009 033 902.
Known heating modules are formed by heating channel sections with an essentially closed radiation space. Heating elements, usually horizontally oriented heat radiators, which emit infrared radiation with a maximum radiation intensity in the range of 800-1500 nm, are arranged on one side of the heating channel. A reflector (primary reflector) provided on this side, behind the heating elements, ensures that radiation emitted by the heating elements toward the rear is reflected back into the heating channel. Reflector elements (counter-reflector and base reflector) are provided on the opposite side wall and on the floor of the radiation space to minimize the heat loss in the heating channel section.
The counter-reflector and base reflector are, for example, polished and/or coated metal elements arranged on the wall and floor, respectively; to withstand the vibrations, etc, which occur during operation in the known machines, they are made with an appropriate degree of mechanical strength.
The problem, however, is that these metal mirrors do not offer optimal reflective properties, which can lead in particular to energy losses and to an undesirable heating of the elements. Dirt on the mirrors and the associated cleaning processes, furthermore, lead to a further deterioration of the reflective properties as a result of scratches, etc.
As an alternative, ceramic elements are known, which have the necessary strength to withstand the mechanical stresses, and which can therefore be used for the counter-reflectors and base reflectors. Such ceramic materials, however, have a comparatively low reflection coefficient.
Ceramic materials which consist primarily of amorphous silicon dioxide and which have a very good diffuse reflectivity, i.e., a comparatively high reflection coefficient, are also known. Such materials, however, suffer from the disadvantage that they have comparatively low mechanical strength and can easily suffer damage during installation or during operation.